MAGNETIC RESONANCE IMAGING WITH ZERO ECHO TIME AND SLICE SELECTION

Abstract

A novel radio frequency sequence, suitable for performing Magnetic Resonance Imaging (MRI) of 2-dimensional (2D) slices of samples exhibiting short magnetization coherence times (i.e., hard tissues). The SS-ZTE pulse sequence contains the following steps: a) magnetizing all spins in the sample from a longitudinal direction to the transverse plane; b) exciting the 2D slice of interest, which comprises the selective locking of said 2D sample slice magnetization while spoiling the magnetization in the rest of sample volume; c) making the magnetization of the selected 2D slice impervious to reconfigurations of the magnetic field gradients from slice selection to encoding and readout; d) reading out of the free induction decay signal of the sample; e) repeating steps (a-d) with different readout directions, so as to gather a corresponding number of radial spokes of the plane defined by the 2D sample slice.

Claims

1. A magnetic resonance imaging, MRI, method comprising a sample set in a magnetic field, B0, along a longitudinal direction so that the magnetization of the sample is initially parallel to said direction of the magnetic field, wherein said method comprises performing an imaging scan of a sample based on a ZTE sequence, the imaging scan comprising the following steps: a) applying a slice selection gradient pulse, G.sub.SS, followed by at least one radio frequency excitation pulse generating a transverse magnetization of the sample with respect to the direction of B0; b) applying a slice selection pulse sequence comprising at least one rf pulse for selective locking of sample magnetization corresponding to a single 2D slice of the sample; c) applying a preservation pulse sequence comprising at least one rf pulse, for keeping magnetization coherence of the selected 2D slice, and additionally comprising the termination of G.sub.SS and the onset of a readout gradient pulse, G.sub.ro; d) performing the encoding, readout, and acquisition of the HD signal of a sample radial spoke defined by G.sub.ro; e) repeating the steps (a-d) along a plurality of readout directions in order to gather a corresponding number of radial spokes in the k-space plane, corresponding to the 2D slice of the sample; f) processing the FID signals gathered in steps a)-e) and reconstructing an image of the 2D slice of the sample.

2. The MRI method according to claim 1, wherein the slice selection pulse sequence comprises a standard spin locking pulse.

3. The MRI method according to claim 1, wherein the slice selection pulse sequence comprises at least one rotary echo SL pulse.

4. The MRI method according to claim 1, wherein both the excitation and slice selection pulse sequences are merged into a homonuclear dipolar-decoupled version of a DANTE selective-excitation sequence.

5. The MRI method according to claim 1, wherein the preservation pulse sequence starts with a hard rf pulse which rotates the transverse component of the magnetization to the longitudinal axis and, once the magnetization is longitudinal, the G.sub.SS gradient is ramped to zero and the readout gradient G.sub.ro is ramped in a perpendicular image-encoding direction; followed by an excitation rf pulse which rotates the sample magnetization towards the transverse plane.

6. The MRI method according to claim 5, wherein the preservation pulse sequence comprises an additional spoiler gradient pulse, interleaved between the termination of G.sub.SS and the onset of G.sub.ro; being this additional spoiler gradient ramped up and then down, to further spoil coherence of magnetization outside the selected 2D slice.

7. The MRI method according to claim 1, wherein the preservation pulse sequence comprises a CHASE sequence.

8. The MRI method according to claim 7, wherein the CHASE sequence is a single CHASE-5 pulse train; and additionally, the G.sub.SS is ramped down linearly before the first 90° pulse and blipped after the last one, while G.sub.ro is first blipped and then ramped up.

9. The MRI method according to claim 5 comprising a dead time, td, before acquisition; along with an acquisition step following a ZTE scheme.

10. The MRI method according to claim 7, wherein td is incorporated into the waiting time between pulses of the preservation pulse sequence; along with an acquisition step following a ZTE scheme.

11. The MRI method according to claim 1, wherein the sample comprises a hard tissue with T2 below 1 ms.

12. The MRI method according to claim 11, wherein the hard tissue comprises bone tissue, and/or tendon tissue.

13. The MRI method according to claim 11, wherein the hard tissue comprises teeth bone tissue.

14. An MRI apparatus comprising: a magnet operable to provide a magnetic field; a rf transmitter configured to transmit a rf field to a sample placed into the magnetic field; a rf receiver arranged to receive a magnetic resonance signal; a data acquisition unit to record the magnetic resonance signals; and a processor for processing the information provided by the data acquisition unit; wherein the rf transmitter is configured to yield ZTE sequences following a method according to claim 1.

15. An MRI apparatus according to claim 14, wherein the rf transmitter and rf receiver are combined in the same physical element.

16. Use of an MRI apparatus according to claim 14 in hard tissue imaging, imaging of archaeological objects, imaging analysis in mineralogy, imaging for gemology certification, soil prospection imaging and imaging analysis of chemical composition of solids.

17. Use of an MRI apparatus according to claim 14 in teeth bone imaging.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be carried out. The accompanying drawings comprise specifically the following figures:

[0039] FIG. 1 is a block diagram showing a SS-ZTE sequence, and including sample radio frequency (rf) pulses, slice selection (G.sub.SS) gradient pulses and readout (G.sub.ro) gradient pulses according to an embodiment of the present invention.

[0040] FIG. 2 displays two possible embodiments of the slice selection (SS) step shown in FIG. 1, either a bare selection locking (SL) pulse (A) or a rotary echo SL pulse (B), the latter being insensitive to rf excitation pulse (B1) inhomogeneities and suppressing in-slice dephasing.

[0041] FIG. 3 shows a possible embodiment of the SS-ZTE sequence shown in FIG. 1, wherein the preservation (P) step comprises a magnetization storage pulse, a gradient spoiling pulse and an excitation pulse.

[0042] FIG. 4 displays a possible embodiment of SS-ZTE sequence shown in FIG. 1, wherein the preservation (P) step includes a CHASE pulse train of length 5, allowing for zero dead time before data acquisition.

[0043] FIG. 5 shows how the slice selection (SS) step is performed applying a bare spin locking pulse with a sample consisting of a test tube filled with a dissolution of copper sulphate.

[0044] FIG. 6 shows how the slice selection (SS) step with the same sample and conditions as in FIG. 5 but replacing the bare spin locking pulse with a rotary echo spin locking pulse.

DETAILED DESCRIPTION OF THE INVENTION

[0045] FIG. 1 is a block diagram showing the SS-ZTE pulse sequence according to an embodiment of the present invention, comprising four main steps: an excitation (E) step, where the whole 3D FoV magnetization is rotated to the transverse plane; an slice-selection (SS) step, where the sample magnetization is selectively locked/spoiled, leaving solely excited the slice of choice; a preservation (P) stage, aimed to make the magnetization and coherence of the selected slice impervious to reconfigurations of the magnetic field gradients; and finally an acquisition (A) step, where the FID signal is detected, recorded and discretized. Every execution of the above procedure (each iteration comprises one execution of all the steps E, SS, P and A) provides data for a single radial spoke in a 2D k-space. For this reason, it is repeated with a plurality of readout (G.sub.ro) directions in order to gather the corresponding number of radial spokes in k-space, with signal contributions coming only from the selected 2D slice of the sample. Then, mathematical tools are employed to reconstruct an image of the sample from the acquired data. Without loss of generality, one can choose x as the direction of the slice selection gradient, such that e.g. only the x=0 2D slice of the sample keeps coherent magnetization. Then, the preservation step is applied with a readout gradient G.sub.ro applied in the y-z plane, such that acquisition gives radial spokes in the k-space k.sub.y-k.sub.z. Mathematical tools are then able to reconstruct the image in y-z of the selected 2D slice.

[0046] In a first aspect, the present invention features the excitation step in a similar way to standard ZTE techniques, i.e., the hard rf excitation is pulsed only after the gradient field (G.sub.ro) has been switched on. The rf pulse in the excitation step must coherently rotate the magnetization by 90° from the longitudinal direction (z) to an axis in the transverse plane, for the subsequent slice-selection step (SS) to work. Therefore, it does not necessarily determine the flip angle θ.sub.x, as later explained in the description of the preservation step (P). It is worth highlighting an important difference between the SS-ZTE and the conventional ZTE sequences: the direction of the slice-selection gradient field (G.sub.SS) during the excitation step (E) eventually determines the slice selected in the former, rather than the readout direction as in the latter.

[0047] With regard to the slice-selection (SS) step, its aim is to selectively lock/spoil the sample magnetization, leaving solely the 2D sample slice of choice coherent.

[0048] In a preferred embodiment of the invention, the slice-selection (SS) step is performed with a bare (standard) spin-locking (SL) pulse, as shown in FIG. 2A. The magnetization in the slice of the FoV for which the SL pulse is resonant will be locked. The spins in the rest of the FoV are not resonantly excited and are therefore subject to dephasing at a rate T2*, accelerated by the presence of the slice-selection gradient field (G.sub.SS). By considering, without loss of generality, that the excitation step leaves the magnetization pointing along −y′, a bare SL pulse of frequency (ωss) and Rabi frequency (Ωss) around −y′ (see FIG. 2A) will lock the spins with Larmor frequencies ω.sub.L detuned by |δv|≲Ω.sub.ss from ωss, while spoiling (i.e. dephasing and spreading out in the transverse plane) the rest.

[0049] In another preferred embodiment of the invention, displayed in FIG. 2B, the slice-selection (SS) steps comprises using a rotary echo SL pulse, where the phase changes from the first to the second half to move the rotation axis from −y′ direction to y′. This makes the SS step insensitive to rf field (B1) inhomogeneities, i.e. to spatial variations of the rf field amplitude and thus Ω.sub.ss. It also partially corrects dephasing for spin in the 2D slice which are not totally resonant to Ωss, improving the quality of the slice.

[0050] In a particularly advantageous embodiment of the invention, both the excitation (E) and slice-selection (SS) steps are merged into a unique homonuclear dipolar-decoupled version of the DANTE selective-excitation sequence. This technique interleaves WAHUHA or other decoupling pulse-sequences, between DANTE excitations which only excite the 2D resonant slice of the sample.

[0051] Meanwhile, the main challenge that SS-ZTE overcomes is how to switch from a gradient configuration which enables slice selection (out of plane) to an orthogonal configuration where a k-space radial spoke can be sampled (in plane), while preserving the magnetization coherence of short T2 samples. This task is performed by the preservation (P) step, which makes the magnetization and coherence of the selected slice impervious to reconfigurations of the magnetic field gradients. In its widest scope, the preservation (P) step comprises a combination of rf and gradient pulses which ensures that only the selected 2D slice of the sample will contribute to the detected FID signal.

[0052] In a preferred embodiment, the preservation (P) step starts with a hard rf pulse that rotates the transverse component of the magnetization to the longitudinal axis z. This MS pulse can be a simple 90° rotation around −x′ (90°.sub.−x′). Once the magnetization is longitudinal, the slice selection gradient (G.sub.SS) is ramped to zero value without dephasing in-slice spins, while dephasing remaining coherences of off-slice spins. The readout gradient (G.sub.ro) is ramped in a perpendicular (image-encoding) direction with regard to G.sub.SS, in order to enable the ulterior acquisition in the corresponding step. Afterwards, an excitation rf pulse rotates the magnetization towards the transverse plane by an angle θ.sub.x (flip angle) which can be chosen different from 90° . This last step is in exact analogy to conventional ZTE.

[0053] Another preferred embodiment of the invention, shown in FIG. 3 for a RT of the SS-ZTE sequence, comprises a preservation (P) step comprising a gradient spoiler pulse (G.sub.ro, spoil) which is interleaved between the termination of the slice selection gradient pulse (G.sub.SS) and the onset of the readout gradient (G.sub.ro). This effectively further spoils (i.e. dephases) the contribution of spins outside the sample which have not completely lost their quantum coherence during the long SL pulse in the slice-selection (SS) step. For instance, this may occur if the sample is composed of a combination of tissues exhibiting both short and long coherence times, for instance, or if SL pulses are intentionally short to limit rf absorption by the sample.

[0054] In another preferred embodiment, the key element in the preservation (P) step is a Combined Hahn And Solid Echo (CHASE) sequence, with G.sub.SS starting on and G.sub.ro starting off, and both following trajectories in such a way that their time integrals after they have switched contribute to the inhomogeneous broadening terms in the Hamiltonian by exactly the same amount before and after the 180° pulses in the CHASE sub-steps, i.e. the time integrals must cancel for each of the spin operators. In this case, the 180° pulses suppress the dephasing otherwise introduced by the dynamic gradient fields, and the 90° pulse trains undo the contribution of dipolar interactions. All 180° and 90° pulses must be quasi-instantaneous (of duration much shorter than T2), and the final flip angle with CHASE is 90°.

[0055] In another preferred embodiment of the invention, summarized in FIG. 4, the CHASE sequence is a single CHASE-5 pulse train and G.sub.SS is ramped down linearly before the first 90° pulse and blipped after the last one, while G.sub.ro is first blipped and then ramped up. Here, the areas A.sub.SS and A.sub.ro must be the same in the ramp and the blip, but A.sub.SS is not necessarily equal to A.sub.ro.

[0056] Finally, the FID signal detection and data acquisition take place in the acquisition (A) step (denoted as a sequence of dots in FIG. 3 and FIG. 4), after the onset of the readout gradient and the termination of the slice selection gradient in the preservation (P) step. As usual in ZTE sequences, the acquisition starts a dead time td after the last rf pulse (see FIG. 3). In the CHASE embodiment, this td coincides with the start of acquisition (A) step, which encodes directly from k=0 in k-space.

[0057] In a preferred embodiment of the invention, the preservation (P) step is implemented with an MS pulse as in FIG. 3, wherein the dead time (td) is long enough to switch the rf electronics from transmission to reception mode, therefore leaving an unsampled gap at the center of k-space; If td is small, a ZTE encoding sequence can be used. If it is too big, the acquisition (A) step can comprise a pointwise k-space center sampling, similarly to standard PETRA technique.

[0058] Another particularly advantageous embodiment of the preservation (P) step is implemented by means of a CHASE pulse, as displayed in FIG. 4, so that td can be incorporated into the waiting time between pulses (τ), thereby leaving no gap at the center of k-space.

[0059] The following paragraphs describe some experimental results obtained through different embodiments of the invention.

[0060] In order to illustrate the slice selection capability of SS-ZTE sequence, FIG. 5 displays an experimental demonstration with a test tube filled with a dissolution of copper sulfate as a sample, set in a reference magnetic field of B0≈0.3 T and a Larmor frequency ωL≈2π.Math.14 MHz. If the excitation (E) step was immediately followed by an acquisition (A) step in which the readout is performed along the slice selection direction (Gro=Gss≈50 mT/m), then the one-dimensional (1D) profile of the reconstruction (measured as spin density) would stand for the full width of the test tube (≈9 mm, see FIG. 5A). Nevertheless, by taking into account a bare SL pulse of Ωss ≈2π.Math.2.5 kHz and duration equal to 500 μs included in the slice-selection (SS) step, it narrows the 1D line profile up to a width of 1 mm (see FIG. 5B).

[0061] Finally, another example of SS-ZTE application is shown in FIG. 6. Here, the previous experiment from FIG. 5 is repeated, but with a rotary echo SL pulse rather than a bare spin locking pulse. FIG. 6A, again, stands for the full width of the test tube, while the 1D line profile boundaries of the selected slice (FIG. 6F) turns out to be sharper in this case, because the effect of applied rf field (B1) inhomogeneities is suppressed by the rotary echo configuration.

[0062] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.